How do I find the externalId of a gradebook columnId

The documentation for extracting grade values with REST says I can identify the columnId via the externalId. I was hoping this would be the same as the name. I could then eliminate the request for all the columns in the course just to find the id of the column I wanted. When that didn’t work, I requested all the information for all the columns. The externalId for the columns was not returned.